I am trying to set the static back pressure for a converging-diverging nozzle in Fluent using the 'Pressure-Outlet' boundary condition. I have put the desired back pressure as the "Gauge Pressure" when I specify the boundary conditions. But when I produce a graph of my results, the static pressure at the outlet according to the graph is not the same as the static pressure that I specified as my boundary condition.

Does anyone know how I can set up my case so my back pressure from the graph equals the back pressure I set for my boundary condition?

since you get the pressure field as solution of RANS therefore fluent does not strictly enforces the outlet condition. Although it try to enforce specified outlet condition, but to extent it does not violate interior solution.

Is there any way that I can make it stick to this back pressure? I am trying to do the same case that I did for an experiment so I can compare the two, but I cant get the boundary conditions to be the same.

i believe that in experiment your boundary is placed at far downstream. Therefore put your boundary at downstream where your boundary condition does not vary, in other words it is far far downstream and therefore does not influenced by flow from nozzle

So there is no way I can specify the static pressure at the outlet plane and run my simulation from this? I have done some tutorials and they have specified the back pressure using the method I have used, so I thought it would be best to follow their method.